There are known sound-absorbing devices having an air duct with a round cross-section covered by a cylindrical sleeve of sound-absorbing material of constant thickness, and for the case of a square cross-section air duct, having differently arranged separating walls of sound-absorbing material.
The sound-absorbing characteristics of the sound-absorbing device are essentially determined by the thickness of the built-in sound-insulating material. It is a problem to damp the noises of deeper (lower) frequency, since this requires a thicker layer of sound-insulating material, thereby causing a reduction of the cross-section of the flow since an increased layer thickness of the sound-insulating material is necessary; this is no solution because of the resulting increase in the air speed, respectively the flow resistance.
Among the known sound-absorbing devices there are types (Soundstream) which have inserts whose shapes change sinusoidically in the direction of the air flow. This solution of the problem has the same limitations because of constriction of the air flow where thicker inserts are used.